1. Field of the Invention
This invention relates to an improvement of speed control method for a numerical control apparatus.
2. Description of the Related Art
Generally known as a speed control method of a machine tool comprising a rotational axis and designed for being capable of varying an angle of its tool relative to a workpiece is to calculate the distributive shift amount in the same manner as in the case of calculation of the shift amount of the linear shift axis so that each axis can be controlled and driven in accordance with the calculated distributive shift amount.
This method is, for example, applied to a machine tool comprising five axes: a first rotational axis ("b" axis) for allowing a tool to rotate about a horizontal axis passing through a base of the tool, a second rotational axis ("c" axis) for allowing the tool to rotate about a vertical axis passing through the base of the tool, and shift axes X, Y and Z constituting orthogonal three axes, each for moving the tool in a direction orthogonal to other axes. In this case, assuming that a point (Sx, Sy, Sz) and another point (Ex, Ey, Ez) represent start and end points of a shift command with respect to a tool tip, respectively; "F" is a command shift speed; "Sb" and "Eb" represent rotational angles of the first rotational axis at the start and end points of the shift command; "Sc" and "Ec" represent rotational angles of the second rotational axis at the start and end points of the shift command; and ".DELTA.T" is a distribution period of each axis. On the premise that the distribution number of times for each axis is equal to those for others and an overall shift amount per distribution period is expressed in the form of a square root of a summation of square of a distributive shift amount of each axis, a distributive shift amount .DELTA.B of the first rotational axis (b-axis), a distributive shift amount .DELTA.C of the second rotational axis (c-axis), a distributive shift amount .DELTA.Ux of the linear shift axis X, a distributive shift amount .DELTA.Uy of the linear shift axis Y, and a distributive shift amount .DELTA.Uz of the linear shift axis Z, are obtained ny solving the following simultaneous equations (1) and (2); ##EQU1##
However, in the equations (1) and (2), the terms .DELTA.B and .DELTA.C have the dimension of angle. Furthermore, no consideration is given to the length of a tool. Thus, executing pulse distribution by dealing with these terms .DELTA.B and .DELTA.C in the same manner as in the case of the shift amount of a linear shift axis could not realize a control such that the shift speed of a tool tip conforms to the command shift speed.
In other words, such a pulse distribution only provides a means for controlling the drive operation of each axis by executing such a linear interpolation processing as applied only to linear shift axes, on the basis of start and end points of a shift command, rotational angles of respective axes at the start and end points of the shift command, a distribution period and a command shift speed as one control data, with the result that the substantial shift speed of the tool cannot be controlled by the command shift speed itself. On the other hands, in order to realize that the tool tip should move at a desired speed, the command shift speed for realizing the desired shift speed must be determined by taking account of the effects which variables, such as distributive shift amount .DELTA.B and .DELTA.C of the rotational axes, give to the actual shift speed of the tool tip. However, it is very difficult to obtain such a command shift speed with the above conventional method.